24 NORTE AMERICAN BIRDS. 



b\ Tail more or less forked, the outer feather longest. 



c 1 . Tail much more than one-third as long as wing, usually (except in sub- 

 genus Thalasseus) forked for more than one-fifth its total length, 

 the outer feathers narrow and pointed at tips; webs of feet occu- 

 pying more than half the interdigital space. 

 d 1 . Depth of bill at base equal to one-third the length of the exposed 

 culmen ; gonys shorter than outer toe, without claw. 



Gelochelidon. (Page 38.) 

 d*. Depth of bill at base less than one-third the length of the exposed 

 culmen ; gonys longer than inner toe, without claw. 



Sterna. (Page 39,) 

 c 2 . Tail little more than one-third as long as wing, forked for less than one- 

 fifth its total length, the outer feathers broad and rounded at tip ; 

 webs of feet occupying less than half the interdigital space. 



Hydrochelidon. (Page 46.) 

 6 2 . Tail graduated Anous. (Page 47.) 



Genus GAVIA Boie. (Page 23, pi. VIII., fig. 1.) 



Species. 



Adult : Entirely pure white, the shafts of the primaries pale yellowish ; bill yel- 

 lowish ; feet black. Young : Similar to adult, but quills, primary coverts, and tail- 

 feathers each marked with a spot of dusky at tip, the lesser wing-coverts spotted 

 with. same. Length 15.00-19.50, wing about 13.25, culmen 1.40. Sab. Arctic 

 Ocean, south, in winter, along Atlantic coast of North America to Newfoundland 

 and New Brunswick 39. G. alba (Gunn.). Ivory Gull. 



Genus RISSA Leach. (Page 23, pi. VIII., fig. 2.) 



Species. 



Common Characters. — Adults with head, neck, rump, upper tail-coverts, tail, 

 tips of secondaries, and entire lower parts pure white ; mantle bluish gray, the 

 quills varied with white and black ; bill yellowish, feet blackish or bright red in 

 life. Young similar to adults, but hind-neck crossed by a blackish collar or patch, 

 and sometimes (in JR. tridactyla) a blackish patch on lesser wing-coverts and black 

 band across tip of tail. Downy young white, tinged above with bufly and yellowish 

 gray, but without spots or other distinct markings. Eggs 2-5, ovate, or short-ovate 

 olivaceous-white, grayish white, brownish white, or bufly, blotched and spotted 

 with brown and lavender-gray. 



a 1 . Legs and feet black, or dusky. Summer adult : Pure white, the mantle deep 

 pearl-gray ; five outer quills with terminal portion black, this decreasing from 

 about 3.25 on the outer quill to .75 (more or less) on the fifth, the outer web 

 of the first almost wholly black ; the fifth, and sometimes the fourth, tipped 

 with white. Winter adult : Similar, but hind part of head and neck washed 



